: 



GASTEIC DENTAL APPAEATUS. 439 



tating the action of the muscles upon them. Th$ crushers are two 

 broad elongated plates (fig. 228, a), each being about ^th of an inch 

 long, and separated from each other at the mesial line, near which they 

 become much thickened. From each of these plates there proceed late- 

 rally numerous parallel bars (fig. 228, b 6), 

 all of which are somewhat thickened at Flg> 228> 



their inner extremities, where they are at- 

 tached to the plates, whilst at their oppo- 

 site ends they are united with the others of 

 the same side by a curved connecting bar 

 (fig. 228, c c), from the outer sides of which 

 are given off various loops and processes. 

 The three uppermost of these bars are the Gastric dental apparatus of 

 largest, the rest gradually diminishmg in *** *?* 



Size as Well as Strength, till the inferior Ones work; <Z, handle-like processes; 

 ,....,, e, central fixed point. (After 



become almost invisible. p ro f. Williamson.) 



(1135.) Prom the upper extremities of the 



two crushers there project upwards and backwards two slender pro- 

 longations (fig. 228, d), which are united by a sort of double hinge 

 near this apex, where they not only play upon each other, but also on a 

 small central fixed point (fig. 228, e) lodged in a little conglobate cel- 

 lular mass. Ehrenberg only describes three transverse bars on each 

 side, which he regards as teeth ; it is obvious that he has only noticed 

 the three upper and larger pairs. It is equally evident that these 

 transverse teeth, as he terms them, do not move upon the strong longi- 

 tudinal plates, as he imagines, but are firmly united with them. Muscles 

 are either attached to the divergent peripheral processes, or to the 

 cellular mass in which these processes are imbedded, causing the entire 

 apparatus to separate along the mesial line by means of the hinge- 

 joint (fig. 228, e), the so-called teeth merely transmitting the motor 

 force to the two longitudinal plates. These latter appendages are thus 

 made to play upon each other with great power, and to act as efficient 

 crushers, bruising the food before it passes into the stomach, as is the 

 case with the gastric teeth of the Crustacea. 



(1136.) From the above remarks it will be seen that, though in its 

 construction the dental apparatus is more complex than is represented 

 by Ehrenberg, in its mode of working it is less so. The conglobate 

 organ in which this apparatus is imbedded is transparent, and composed 

 of numerous large cells, each of which contains a beautiful nucleus with 

 its nucleolus. The cells are only seen when the organ is ruptured be- 

 tween two plates of glass, when they readily separate from one another ; 

 but the nuclei, with their contained nucleoli, are distinctly visible in 

 the living animal. Delicate muscular threads most probably penetrate 

 this organ to reach the dental apparatus ; but Professor Williamson has 

 hitherto been unable to detect their presence satisfactorily. 



